Published patent application US2009/0322208A1 discloses a light emitting device. A Light Emitting Diode (LED) is provided within a conical cavity formed by a recessed housing. At the front side of the recessed housing the conical cavity is covered with a transparent thermal conductor layer on which a refractory phosphor layer is provided. At the backplane of the recessed housing is provided a heat sink and the side walls of the recessed housing are covered with a metal frame. The conical cavity may be filled with a material such as silicone.
The LED emits light towards the phosphor layer. A part of the light of the LED is converted by the phosphor layer into light of another color. Although the conversion of the color of light is relative efficient, still some energy is dissipated by the phosphor layer. Typically 20 to 30% of the energy of the converted light is lost due to Stokes shift induced by the phosphor layer and about 2 to 20% due to limited quantum efficiency. Especially when high power LEDs are used, the phosphor layer may become relatively hot which results in the degradation of the efficiency of the phosphor layer. The heat, which is generated in the phosphor layer of the light emitting device, is conducted by the transparent thermal conductor layer towards the metal frame which, subsequently, drains away the heat towards the heat sink.
Although the light emitting device provides the metal frame to conduct the heat away from the phosphor layer, the central area of the phosphor layer, which is an area of the phosphor layer that is located furthest away from the metal frame, becomes still relatively hot.